The anti-Islamic State fight could devolve into a war between the Kurds and Ankara and disrupt the plan to take back the terrorist group’s de facto capital.

ANKARA, Turkey — The United States and Turkey are on a collision course in Raqqa, as Turkish officials warn that Washington’s reliance on Kurdish forces to liberate the Islamic State’s de facto capital would severely damage its relationship with Ankara.

The current U.S. plan to advance on Raqqa depends heavily on the People’s Protection Units (YPG), a Syrian Kurdish militia that Washington has supported with airstrikes and provided with military equipment. But Turkish officials accuse the group of being just another name for the Kurdistan Workers’ Party (PKK), a group listed as a terror organization by Washington and Ankara that has waged a decades-long guerrilla war against Turkey.They say that the PKK has used YPG-held territory in Syria — territory gained in part with the backing of the United States — to train their fighters and plan attacks against Turkey.

“If [the United States] insists on carrying on this operation with terror organizations, our relations will be harmed – that is clear,” Prime Minister Binali Yildirim told a group of visiting journalists this month. “Because it will show that they value terror organizations more than us.”

Turkish officials repeatedly declined to specify the steps they could take if Washington maintains its alliance with the YPG. But if Ankara wanted to, it could throw a sizeable wrench in U.S. strategy in the region — for example, by cutting off access to airbases in southern Turkey, from which the United States launches airstrikes in Syria and Iraq, or by deepening its cooperation with Russia.

President Donald Trump’s administration is currentlyengaged in an intense debate over whether to continue supporting Kurdish forces’ advance on Raqqa, or shift U.S. support to Turkey and its allies. Top American commanders view the Kurds as superior fighters and the only viable option for ousting the Islamic State. They are skeptical of Turkey’s competing proposal to exclude the Kurds altogether and let Turkish troops and an Ankara-backed Syrian Arab force retake the ISIS stronghold, with U.S. help.

Even as the Trump administration weighs its options, the U.S. military is ramping up for the assault, drawing up plans to deploy up to 1,000 more American soldiers to Syria in support of the YPG and allied forces, known collectively as the Syrian Democratic Forces (SDF), which have advanced mere miles from the city. Pentagon officials assess that the roughly 27,000 Kurds in the 50,000-strong SDF are the more effective, experienced fighters.

“The SDF is the partner force most capable of acting swiftly to isolate Raqqa,” said Defense Department spokesman Eric Pahon.

The White House did not respond to request for comment on their view of the potential diplomatic collision with Turkey.

Pahon said that a majority of the SDF force currently isolating Raqqa are Syrian Arab fighters, including a significant contingent that hail from the area around the city. Nevertheless, he said, it’s not clear which group will actually liberate the Islamic State stronghold. “While that isolation is underway, we will continue to plan for the subsequent phases with our allies and partners, including Turkey,” he said. “There have been no decisions made on what force will be used to liberate Raqqa.”

For Turkey, this is not solely a foreign policy issue. Turkish officials claim that by backing the YPG, the United States is directly undermining their domestic security.

“We have evidence that proves a clear connection between the PKK and PYD-YPG establishment in Syria,” said Ibrahim Kalin, a foreign policy advisor to President Recep Tayyip Erdogan, using the acronym for the Syrian Kurdish party affiliated with the YPG. “Some of the suicide bombers in Turkey last year were trained in and came from the region under PYD-YPG rule.”

Turkeyblamed a February 2016 car bombing in Ankara on a fighter trained in areas under YPG control, and shelled the group’s positions in Syria in reprisal. A paper for West Point’s Combating Terrorism Center published last year alsoclaimed that a PKK offshoot was using YPG-held territory as a “key launching pad” for attacks in Turkey.

These claims are hotly debated by U.S. officials. The U.S. military maintains that the Syrian Kurdish force is no threat to Turkey. Meanwhile, Pahon said that Washington “will continue to support Turkey in its fight against the PKK.”

However, other current and former officials have engaged in a long-running debate over the Turkish role in Syria, and the wisdom of relying on the YPG.

Derek Chollet, who served as assistant secretary of defense in President Barack Obama’s administration, said that he worked “constantly” to get the United States and Turkey on the same page – but was never able to assuage Ankara’s concerns about the YPG.

“The U.S. does not share Erdogan’s view … that the Syrian Kurds and PKK are somehow the same,” he said. “The U.S. sees Syrian Kurds as good partners.” But Turkish officials “have neuralgia about our cooperation with the Kurds,” he said.

Fred Hof, who served as Obama’s special envoy for Syria in 2012 after working on the issue for years, gives some credence to Turkish claims. The YPG, he said, “is essentially the Syrian affiliate of the PKK.” Meanwhile, he faulted the Obama administration for failing to incorporate Ankara into its Syria strategy, saying, “there was just not a strong attempt to build a relationship and trust and confidence with Turkey.”

But both warn that the Trump administration may not have the luxury of time. As a candidate, Trump criticized the Obama administration’s approach to the war against the Islamic State, promising to “bomb the hell out of” the jihadist group.

“If [Trump] were to wait, to pump the breaks on this alternative solution to better satisfy the Turks, it would be tough to explain how this comports with his campaign rhetoric,” said Chollet, who advised his campaign rival, Hillary Clinton.

Hof noted that the longer U.S. officials wait to launch the assault on Raqqa, the more time Islamic State leadership has to plot more attacks against the West.

“How lucky do we think we are?” Hof said. “Raqqa is the place where operations in Paris, Brussels, and various parts of Turkey were all planned.”

But even if the United States ends up supporting the SDF’s assault on Raqqa, the dispute with Turkey could still derail the assault. Turkish troops and their Syrian allies have recently tried to advance on the town of Manbij, west of Raqqa, as the next step in their campaign to push Kurdish forces away from their border. Officials with the Kurdish-led coalition warn that they may need to divert forces away from Raqqa if tensions escalate further with Ankara.

“The situation has gotten very complicated, and we’re on the verge of a fierce war,” said Ilham Ahmed,the co-chair of the SDF’s political wing.“We are still getting support from the Americans, but if the Turkish attacks continue, we won’t be able to focus our efforts on Raqqa.”

The Kurdish-led forces’ fight with Turkey has also led them to pursue a tentative rapprochement with Syrian strongman Bashar al-Assad. Earlier this month, these groupsinvited the Syrian government totake control of several villages near the front line with Turkey – apparently seeing the Syrian government presence as a potential buffer against a Turkish advance.

“We believe that the use of terms such as opposition or regime supporters, despite its importance, it has to be put on the back burner,” said Sihanouk Dibo, a senior presidential advisor to the PYD.

On the other side of Syria’s battle lines, Abu Waleed, a commander with the Turkish-backed Sultan Mourad brigades, is preparing for the coming war with the Kurdish forces. He speaks warmly of the “historic stance” Turkey took in favor of the Syrian uprising, saying that it is the “only one that is standing with the Free Syrian Army.”

Abu Waleed is watching the international politicking avidly, knowing that it could mean the difference between victory and defeat. But however it turns out, he believes that his forces are barreling toward Raqqa, whether Washington likes it or not.

The fight would be easier, he acknowledges, with the United States on his side. He looked back with nostalgia on the days his brigade received U.S. support – and expressed confusion over why the partnership ended.

“We had a very good team here, the Pentagon team. We stayed together, we ate and drank together, and they saw the suffering of our people here,” he said. “[Then] they said they wanted to leave — we said no problem, and they left … and they got a new team that unfortunately didn’t join us here, they went to the PKK.”